1. Field of the Invention
The present invention relates to a sheet advancing device for use in a typewriter or the like, and more particularly to such device with an improved detention mechanism.
2. Description of the Prior Art
FIG. 1 shows a conventional sheet advancing device, in which a platen 1 functioning as a sheet advancing roller is provided, at an end of a shaft 2 thereof, with a manual sheet advancing knob 3.
In this example a printing sheet wraps the platen over about a half of the periphery thereof and is advanced by the rotation of the platen.
On said platen shaft 2 there is provided a pulley 4 for driving by a sheet advancing motor 5 through a pulley 6 mounted on the shaft thereof and a timing belt 7 linking said pulleys. Also on the platen shaft 2 there is provided a detention disk between the pulley 4 and the sheet advancing knob 3. The detention disk 8 is star-shaped, with plural projections and recesses along the periphery.
In the vicinity of the detention disk 8, a detention lever 9 is rotatably supported at an end thereof by a shaft 10 fixed to the body of the apparatus, and is maintained in contact with the periphery of the detention disk 8 by a roller 9a at the other end. A spring 11 is provided between the detention lever 9 and the body of the apparatus to bias the detention lever 9 anticlockwise as indicated by an arrow shown in FIG. 1, thus pressurizing the roller 9a against the detention disk 8.
In case of manual sheet advancing in the above-described mechanism, the knob 3 is manually rotated causing the projecting parts of the star-shaped detention disk 8 to pass under the detention lever 9 against the force of the spring 11 and thus advancing the sheet with clicking motions.
On the other hand, in case of automatic sheet advancing, the motor 5 has to rotate the platen 1 by pushing up the detention lever 9 and is therefore required to have a power equal to the sum of sheet advancing force and detentive force.
For achieving securer click motion in such mechanism, the spring 11 of the detention lever 9 has to be made stronger to rotate, by the spring, not only the detention disk 8 but also the sheet advancing motor 5.
However, if the spring 11 of the detention lever 9 is made stronger in this manner, the sheet advancing motor 5 has to have a stronger power for pushing up the detention lever 9 as explained before, so that there is required a larger force to rotate the motor when it is not energized.
Stated otherwise, the use of a stronger power of the motor 5 leads to a correspondingly stronger linkage with other component parts, which apply a stronger rotating power to the motor when it is not energized.
It is therefore necessary to further strengthen the spring 11 of the detention lever 9, and the power of the motor 5 has to be correspondingly increased. In this manner the use of a considerably strong motor 5 and a correspondingly large electric power for driving such motor are required for ensuring secure detentive function.
FIG. 2 shows another conventional sheet advancing device, in which a sheet advancing shaft 101 is provided with a sheet advancing roller 102 made for example of rubber.
A slidable ring 103 is rotatably fitted on the shaft 101 but is prevented from rotary motion since a pin 105 fixed on the shaft 101 engages with a recess 103a of the ring 103.
A spring 104 is positioned between the slidable ring 103 and the sheet advancing roller 102 to bias the slidable ring 103 in a direction away from the sheet advancing roller 102.
The slidable ring 103 is provided, on a side thereof opposite to the sheet advancing roller 102, with a ratchet 103b, and is further provided with an integral cylindrical part 103c which is slidably fitted on the shaft 101 and on which a pulley 108 is rotatably fitted.
The pulley 108 is provided, on a side thereof facing the sheet advancing roller 102, with a ratchet 109 for engaging with the aforementioned ratchet 103b.
The ratchets 103b, 109 are normally in mutually engaging position since the slidable ring 103 is biased by the spring 104.
The pulley 108 is provided, on the other side thereof, with an integral detention gear 110 having projections and recesses at a regular interval along the periphery, and a detention spring 111, which is fixed at an end to the apparatus, is fitted at the other free end with one of the recesses.
The detention spring 111 is designed to have a considerably strong pressure in order to prevent undesired displacement of the sheet.
The cylindrical part 103c of the slidable ring 103 is provided, at the outer end, with a knob 112, which is fitted on the outer end of the shaft 101 in axially slidable but non-rotatable manner.
A sheet advancing stepping motor 106 is provided, on the shaft thereof, with a pulley 107, which is linked with a belt 107a with the aforementioned pulley 108 for transmitting the rotating force.
The sheet advancing function in the above-described structure is conducted in the following manner.
In case of automatic sheet advancement, a control signal is supplied to the stepping motor 106 for causing rotation of a determined angle, which is transmitted through the belt 107a to the pulley 108.
Since the ratchets 103b and 109 are normally in the mutually engaging state, the rotation of the pulley 108 is transmitted to the slidable ring 103, whereby the shaft 101 and the sheet advancing roller 102 are rotated by the pin 105 to advance the sheet.
In case of manual advancement the knob 112 is depressed whereby the integral slidable ring 103 is pressed inward against the function of the spring 104. Thus the slidable ring 103 is moved toward the roller 102 and the ratchet 103b is disengaged from the ratchet 109.
If the knob 112 is rotated in this state, the sheet is advanced manually by the rotation of the roller 102 through the pin 105 but the pulley 108 remains stopped.
However, in the above-described structure in which the detention spring 111 contacts the detention gear 110 with a strong force in order to prevent displacement of the sheet, the stepping motor 106 has to be large and of a high power in order to advance the sheet against the pressure of the contact, and the power supply unit has to be accordingly large and expensive.
In addition the use of such a high-powered motor requires the use of an expensive resistant material for the detention gear 110 in order to prevent abrasion thereof.